Blow-molded foam and process for producing the same

ABSTRACT

An object of the invention is to provide a blow-molded foam which has homogeneous foamed cells in size, is light in weight, and is excellent in surface smoothness, and a process for producing the same. The invention is directed to a blow-molded foam  1  having a wall portion formed in such a manner that a thermoplastic resin containing a foaming agent mixed therewith is subjected to blow molding. Herein, the wall portion has a closed cell structure in which a plurality of foamed cells are contained. The wall portion has an expansion ratio of not less than 2.0 times. The wall portion has an outer face having a center-line average surface roughness Ra of less than 9.0 μm. The foamed cell has a cell diameter having a standard deviation of less than 40 μm in a thickness direction of the wall portion.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of Ser. No. 12/935,520, filed Nov. 10,2010, which is a national phase of PCT/JP2009/001519 filed Mar. 31,2009, and claims priority from, Japanese Application Number 2008-093894filed Mar. 31, 2008.

TECHNICAL FIELD

The present invention relates to a blow-molded foam and a process forproducing the same.

BACKGROUND ART

A foam blow-molding process is carried out by extruding a thermoplasticresin containing a foaming agent added thereto into an atmosphere as aparison and inserting the parison between split mold blocks (e.g., referto Patent Document 1).

As a blow-molded foam product obtained by such a process, there has beenknown one that principally contains a polypropylene-based resin havingpredetermined physical properties (e.g., refer to Patent Document 2).

In such a blow-molded foam product, however, when being released to theatmosphere, foamed cells in the parison are abruptly expanded to makecells larger. Occasionally, there is a possibility that the cells areruptured.

In order to overcome this disadvantage, there has been examined aprocess for making foamed cells finer in diameter while maintaining ahigh expansion ratio.

For example, there has been mentioned a foam duct molded by foamblow-molding in which a supercritical fluid is added as a foaming agent(e.g., refer to Patent Document 3).

-   Patent Document 1: JP 63-309434 A-   Patent Document 2: Japanese Patent No. 3745960-   Patent Document 3: JP 2005-241157 A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

As described in Patent Document 3, in the foam duct having the finefoamed cells, the diameters of the respective foamed cells are madesmaller, but are different in size from one another. Consequently, theresulting foam duct fails to provide satisfactory smoothness on asurface thereof.

The present invention has been devised in view of the circumstancesdescribed above, and an object thereof is to provide a blow-molded foamwhich has homogeneous foamed cells in size, is light in weight, and isexcellent in surface smoothness, and a process for producing the same.

Solutions to the Problems

The present inventors have eagerly conducted studies for solving theproblems described above and, as a result, have found out that thefollowing configurations are allowed to solve the problems. Thus, thepresent inventors have completed the present invention.

That is, the present invention relates to (1) a blow-molded foam havinga wall portion formed in such a manner that a thermoplastic resincontaining a foaming agent mixed therewith is subjected to blow molding,wherein the wall portion has a closed cell structure in which aplurality of foamed cells are contained, the wall portion has anexpansion ratio of not less than 2.0 times, the wall portion has anouter face having a center-line average surface roughness Ra of lessthan 9.0 μm, and the foamed cell has a cell diameter having a standarddeviation of less than 40 μm in a thickness direction of the wallportion.

The present invention also relates to (2) a blow-molded foam accordingto the configuration (1), wherein the thermoplastic resin is apolyolefin-based resin.

The present invention also relates to (3) a blow-molded foam accordingto the configuration (2), wherein the polyolefin-based resin is apropylene homopolymer having a long-chain branched structure.

The present invention also relates to (4) a blow-molded foam accordingto any one of the configurations (1) to (3), wherein an average celldiameter of the foamed cell in the thickness direction of the wallportion is less than 300 μm.

The present invention also relates to (5) a blow-molded foam accordingto any one of the configurations (1) to (3), wherein an average celldiameter of the foamed cell in the thickness direction of the wallportion is less than 100 μm, and the standard deviation of the celldiameter of the foamed cell in the thickness direction of the wallportion is less than 30 μm.

The present invention also relates to (6) a blow-molded foam accordingto any one of the configurations (1) to (5), which is a climate controlduct for vehicles.

The present invention also relates to (7) a process for producing theblow-molded foam according to any one of the configurations (1) to (6),including: a mixing step of adding a thermoplastic resin to a foamingagent and mixing the thermoplastic resin with the foaming agent in anextruder to prepare a resin mixture; a retaining step of retaining theresin mixture at a cylinder-shaped space defined between a mandrel and adie outer cylinder; an extruding step of extruding a parison from a dieslit by use of a ring-shaped piston; and a molding step of subjectingthe parison to blow molding by clamping split mold blocks with theparison inserted therebetween and blowing air on the parison.

The present invention also relates to (8) a process according to theconfiguration (7), wherein the thermoplastic resin is a polyolefin-basedresin, and the foaming agent is in a supercritical state.

The present invention also relates to (9) a process according to theconfiguration (7) or (8), wherein in the extruding step, the parison isextruded at an extrusion rate of not less than 700 kg/hr.

Effects of the Invention

In the blow-molded foam according to the present invention, the wallportion has the closed cell structure in which the plurality of foamedcells are contained and also has the expansion ratio set within apredetermined range. Thus, it is possible to reduce the weight of theblow-molded foam. Moreover, the standard deviation of the cell diameterof the foamed cell in the thickness direction of the wall portion is setwithin a predetermined range. Thus, it is possible to make the foamedcells homogeneous in size. Further, the center-line average surfaceroughness Ra of the outer face of the wall portion is set within apredetermined range. Thus, it is possible to achieve high surfacesmoothness.

Therefore, for example, in a case where the blow-molded foam is used asa climate control duct for vehicles, it is possible to reduce frictionalresistance against flowing air and to improve ventilation efficiency.Thus, it is possible to reduce pressure loss of climate control air andto reduce a possibility that condensed moisture adheres to an outside ofa wall face of the duct.

Moreover, in a case where the blow-molded foam is used as a skinnedpanel, it is possible to improve a deposition strength of a reinforcingrib to be formed on an inside of a wall face of the panel and adeposition strength of a skin to be bonded to an outside of the wallface of the panel. Further, it is also possible to provide high rigidityand excellent appearance.

In the case where the thermoplastic resin is a polyolefin-based resin,the blow-molded foam is improved in impact resistance because thismaterial is excellent in flexibility. Preferably, the polyolefin-basedresin is a propylene homopolymer having a long-chain branched structure.In this case, the resin is foamed with ease, so that the resultingfoamed cells are made homogeneous.

In the case where the average cell diameter of the foamed cell in thethickness direction of the wall portion is less than 300 μm, theblow-molded foam is further improved in surface smoothness. Morepreferably, the average cell diameter is less than 100 μm.

In the process for producing a blow-molded foam according to the presentinvention, the resin mixture is retained at the predetermined position.Thus, it is possible to homogenize foamed cells in size. Moreover, theresin mixture is extruded at the predetermined extrusion rate by use ofthe ring-shaped piston. Thus, it is possible to subject the resinmixture to blow molding with the size of the foamed cell maintained.

Accordingly, the process for producing a blow-molded foam allowsproduction of a blow-molded foam which has homogeneous foamed cells insize, is light in weight and is excellent in surface smoothness.Moreover, the foamed cells are made finer by use of a foaming agentwhich is a supercritical fluid.

In the process for producing a blow-molded foam, when the extrusion rateof the parison is not less than 700 kg/hr in the extruding step, thefoamed cells are further homogenized in size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view that illustrates a blow-molded foamaccording to a first embodiment of the present invention.

FIG. 2 is a flowchart that illustrates a process for producing ablow-molded foam according to the present invention.

FIG. 3 is a partial section view that illustrates an extrusion head foruse in the process for producing a blow-molded foam according to thepresent invention.

FIG. 4 is a section view that illustrates a method of blow molding inthe process for producing a blow-molded foam according to the presentinvention.

FIG. 5 is a perspective view that illustrates a blow-molded foamaccording to a second embodiment of the present invention.

FIG. 6 is a section view that illustrates the blow-molded foamillustrated in FIG. 5.

FIG. 7 is an enlarged photograph, which is taken by a CCD camera, of across section of a wall face of a sample in Example 1.

FIG. 8 is an enlarged photograph, which is taken by a CCD camera, of across section of a wall face of a sample in Comparative Example 1.

FIG. 9 is a partial section view that illustrates a conventionalextrusion head.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described indetail below with reference to the attached drawings if necessary. Inthe drawings, identical reference signs designate identical components,and duplicate description is not given here. Moreover, positionalrelations such as a up-to-down relation and a right-to-left relation aredetermined based on the positional relations illustrated in thedrawings, unless otherwise specified. In the respective drawings,further, dimensional ratios are not intended to be limited to ratiosillustrated in the drawings.

First Embodiment

The following description is given about a case where a blow-molded foamaccording to a first embodiment of the present invention is used as aclimate control duct.

FIG. 1 is a perspective view that illustrates the blow-molded foamaccording to the first embodiment of the present invention.

As illustrated in FIG. 1, the blow-molded foam 1 (hereinafter, alsoreferred to as “climate control duct”) according to this embodiment hasa wall portion formed in such a manner that a thermoplastic resincontaining a foaming agent mixed therewith is subjected to blow molding.The climate control duct 1 includes a main body 11, an air inlet 13formed at a first end of the main body 11, and an air outlet 12 formedat a second end of the main body 11. Details of the blow molding will bedescribed later. Moreover, the air outlet 12 is opened by cutting aportion which is closed by post-processing after the blow molding.

The climate control duct 1 has a hollow structure in which a crosssection is has a rectangular shape. That is, the cross section of themain body 11 has a hollow structure surrounded with the wall portion.

Therefore, the climate control duct 1 is allowed to guide climatecontrol air to the hollow portion.

In the climate control duct 1, the main body 11 is curved smoothly, andhas such a function that the climate control air that has been allowedto flow into the air inlet 13 is made to flow out from the air outlet 12that is directed in an “L”-shaped direction relative to a direction inwhich the climate control air has been allowed to flow in.

For example, in a climate control duct for vehicles, the air inlet 13 iscoupled to an air conditioner unit, and climate control air suppliedfrom the air conditioner unit is allowed to flow through the hollowportion, and can be discharged from the air outlet 12 that is disposedat a desired position.

The wall portion has a closed cell structure in which a plurality offoamed cells are contained. Herein, the closed cell structure refers toa structure having a plurality of foamed cells, and means a structure inwhich at least a closed cell ratio is not less than 70%.

In the climate control duct 1, the wall portion has the closed cellstructure, so that the climate control duct 1 is excellent in surfacesmoothness and outer appearance. In particular, the climate control ductis advantageous in improving ventilation efficiency and in reducinggeneration of condensed moisture.

The foamed cells are preferably designed to have an average celldiameter of less than 300 [μm in a thickness direction of the wallportion, more preferably, less than 100 μm. Herein, the average celldiameter refers to an average value of the maximum diameters of therespective cells in the thickness direction of the wall portion.

When the average cell diameter is not less than 300 μm, the surfaceroughness becomes large in comparison with a case where the average celldiameter falls within the range described above, and tends to causedegradation in surface smoothness.

In the climate control duct 1, the average wall thickness of the wallportion is preferably not more than 3.5 mm.

When the average wall thickness exceeds 3.5 mm, an air passage isnarrowed in comparison with the average wall thickness that falls withinthe range described above, and tends to cause degradation in ventilationefficiency.

In the climate control duct 1, the outer face of the wall portionpreferably has a center-line average surface roughness Ra of less than9.0 μm, more preferably, less than 6.0 μm. Herein, the center-lineaverage surface roughness Ra is a value measured in conformity with JISB0601.

When the center-line average surface roughness is set to less than 9.0μm, it is possible to provide excellent surface smoothness and outerappearance. In particular, the resulting climate control duct isadvantageous in improving ventilation efficiency and in reducinggeneration of condensed moisture.

In the climate control duct 1, the standard deviation of cell diameterof the foamed cell in the thickness direction of the wall portion isless than 40 μm. Herein, the standard deviation of the cell diameterindicates the evenness of the foamed cell diameters, and the smaller thestandard deviation, the more even foamed cell diameter is provided.

When the standard deviation of the cell diameter exceeds 40 μm,deviations in the foamed cell diameters become greater. Consequently,the surface smoothness and outer appearance tend to become deteriorated.The standard deviation of the cell diameter is more preferably less than30 μm.

In the climate control duct 1, the expansion ratio of the wall portionis not less than 2.0 times. Herein, the expansion ratio is a valueobtained by dividing the density of a thermoplastic resin used upon thefoam blow-molding by the apparent density of the wall face of theblow-molded foam.

When the expansion ratio is less than 2.0 times, it is not possible toobtain a lightweight blow-molded foam.

The blow-molded foam 1 (climate control duct) according to thisembodiment is obtained in such a manner that a thermoplastic resincontaining a foaming agent mixed therewith is subjected to blow molding.

Examples of such a thermoplastic resin include polyolefin-based resinssuch as a polyethylene resin and a polypropylene resin. Since thepolyolefin-based resin is excellent in flexibility, it is possible toimprove impact resistance of the blow-molded foam.

Among these, the thermoplastic resin preferably has a propylene unit,and specific examples thereof include a propylene homopolymer, anethylene-propylene block copolymer, an ethylene-propylene randomcopolymer, and the like.

Moreover, among these, the propylene homopolymer having a long-chainbranched structure is more preferably used. In this case, the resin isfoamed with ease since its melt tension becomes higher, and foamed cellsare further homogenized.

The propylene homopolymer having a long-chain branched structure ispreferably prepared as a propylene homopolymer having a weight-averagebranching index of not more than 0.9. Moreover, the weight-averagebranching index g′ is represented by V1/V2, and V1 represents a limitingviscosity number of a branched polyolefin, while V2 represents alimiting viscosity number of a linear polyolefin having the sameweight-average molecular weight as that of the branched polyolefin.

As the thermoplastic resin, a polypropylene resin having a melt tensionin a range from 30 to 350 mN at 230° C. is preferably used. Herein, themelt tension refers to a fused-state tension. When the melt tensionfalls within the range, the foaming polypropylene-based resin exhibits astrain hardening characteristic, and is allowed to provide a highexpansion ratio.

The thermoplastic resin preferably has a melt flow rate (MFR) in a rangefrom 1 to 10 at 230° C. Herein, the MFR is a value measured inconformity with BS K-7210.

When the MFR is less than 1, it tends to become difficult to raise theextrusion rate in comparison with the case where the MFR falls withinthe range. When the MFR exceeds 10, it tends to become difficult tocarry out blow molding due to occurrence of drawdown or the like incomparison with the case where the MFR falls within the range.

A styrene-based elastomer and/or a low-density polyethylene arepreferably added to the thermoplastic resin. By adding the styrene-basedelastomer or the low-density polyethylene to the thermoplastic resin, itis possible to improve the impact strength of the blow-molded foam at alow temperature.

Although not particularly limited, any elastomer may be used as thestyrene-based elastomer, as long as it has a styrene unit with hydrogenadded to its molecule. Examples thereof include hydrogenated elastomerssuch as a styrene-ethylene butylene-styrene block copolymer, astyrene-ethylene propylene-styrene block copolymer, and astyrene-butadiene random copolymer. The compounding ratio of thestyrene-based elastomer is preferably set within a range of less than 40wt % with respect to the thermoplastic resin.

Moreover, the content of styrene in the styrene-based elastomer ispreferably less than 30 wt %, more preferably less than 20 wt %, fromthe viewpoint of impact strength at a low temperature.

As the low-density polyethylene, from the viewpoint of impact strengthat a low temperature, those having a density of not more than 0.91 g/cm³are preferably used. In particular, a linear ultra-low densitypolyethylene, polymerized by a metallocene-based catalyst, is preferablyused.

The compounding ratio of the low-density polyethylene is preferably in arange less than 40 wt % with respect to the thermoplastic resin.

The thermoplastic resin is foamed by using a foaming agent before beingsubjected to blow molding.

Examples of the foaming agent include inorganic foaming agents such asair, carbon dioxide gas, nitrogen gas and water, or organic foamingagents such as butane, pentane, hexane, dichloromethane anddichloroethane.

Among these, air, carbon dioxide gas or nitrogen gas is preferably usedas the foaming agent. In this case, it becomes possible to preventtangible matters from being mingled therein, and consequently tosuppress degradation of durability and the like.

Moreover, as the foaming method, a supercritical fluid is preferablyused. That is, carbon dioxide gas or nitrogen gas is preferably broughtinto a supercritical state, and allowed to foam the resin mixture. Inthis case, it is possible to uniformly generate cells positively.

In addition to the styrene-based elastomer, low-density polyethylene andfoaming agent, a core agent, a colorant and the like may be added to thethermoplastic resin.

In the climate control duct 1 (blow-molded foam) according to thisembodiment, the wall portion has the closed cell structure in which theplurality of foamed cells are contained. When the expansion ratio of thewall portion is set within the predetermined range, it is possible toprovide a lightweight structure. When the standard deviation of the celldiameter of the foamed cell in the thickness direction of the wallportion is set within the predetermined range, it is possible to providehomogeneous foamed cells in size. When the center-line average surfaceroughness Ra on the outer face of the wall portion is set within thepredetermined range, it is possible to achieve high surface smoothness.

Moreover, the climate control duct 1 has a low frictional resistanceagainst flowing air, and the ventilation efficiency can be improved.Thus, the pressure loss of climate control air is reduced, therebymaking it possible to reduce generation of condensed moisture on theoutside of the wall face of the duct.

The following description is given about a process for producing theblow-molded foam according to the present invention.

FIG. 2 is a flowchart that illustrates the process for producing theblow-molded foam according to the present invention.

As illustrated in FIG. 2, the process for producing a blow-molded foamaccording to this embodiment includes a mixing step S1 of adding athermoplastic resin to a foaming agent and mixing the thermoplasticresin with the foaming agent in an extruder to prepare a resin mixture,a retaining step S2 of retaining the resin mixture at a cylinder-shapedspace defined between a mandrel and a die outer cylinder, an extrudingstep S3 of extruding a parison from a die slit by use of a ring-shapedpiston, and a molding step S4 of subjecting the parison to blow moldingby clamping split mold blocks with the parison inserted therebetween andblowing air on the parison.

In the process for producing a blow-molded foam according to thisembodiment, since the parison is extruded at a predetermined extrusionrate by using the ring-shaped piston, the blow molding is carried out,with the size of the foamed cells being maintained.

With this arrangement, the process for producing the blow-molded foammakes it possible to obtain a blow-molded foam which has homogeneousfoamed cells in size, is light in weight, and is excellent in surfacesmoothness.

The following description is given about the respective steps in detail.

(Mixing Step)

The mixing step S1 is a step of adding a thermoplastic resin to afoaming agent and mixing the thermoplastic resin with the foaming agentin the extruder to prepare a resin mixture. In this case, any one ofconventionally known machines may be used on demand as the extruder.

Moreover, in the process for producing the blow-molded foam according tothis embodiment, the polyolefin-based resin described above is used asthe thermoplastic resin, and the foaming agent is used in itssupercritical state. By using the foaming agent that is a supercriticalfluid, the foamed cells are made finer.

Herein, the foaming agent is preferably prepared as a carbon dioxide gasor a nitrogen gas. These gases can be made into a supercritical stateunder comparatively moderate conditions.

More specifically, in the case where the carbon dioxide gas is used asthe supercritical fluid, the following conditions are set: the criticaltemperature is 31° C. and the critical pressure is not less than 7.4MPa. In the case where the nitrogen gas is used as the supercriticalfluid, the following conditions are set: the critical temperature is149.1° C. and the critical pressure is not less than 3.4 MPa.

Moreover, by allowing the polyolefin-based resin to foam by using thesupercritical fluid, a resin mixture is obtained. At this time, asdescribed above, a styrene-based elastomer and/or a low-densitypolyethylene may be added to the polyolefin.

(Retaining Step)

The retaining step S2 is a step of retaining the resin mixture at acylinder-shaped space defined between a mandrel and a die outercylinder. The retaining step is carried out using an extrusion head.

FIG. 3 is a partial section view that illustrates the extrusion head foruse in the process for producing a blow-molded foam according to thepresent invention.

As illustrated in FIG. 3, the extrusion head 20 includes a die outercylinder 28, a mandrel 27 that is placed substantially in the center ofthe die outer cylinder 28, a cylinder-shaped space 29 defined betweenthe die outer cylinder 28 and the mandrel 27, a ring-shaped piston 22for pressing the resin mixture retained at the cylinder-shaped space 29downward, and a die slit 21 for discharging the resin.

In the retaining step S2, the resin mixture extruded by an extruder (notillustrated) moves along the periphery of the mandrel 27, is dropped inthe cylinder-shaped space 29 between the mandrel 27 and the die outercylinder 28, and is retained at the cylinder-shaped space 29.

Herein, the amount of resin to be retained is preferably 5 to 40 liters.

In the process for producing a blow-molded foam according to thisembodiment, since a system in which the resin mixture is retained in thecylinder-shaped space 9 is prepared, the sizes of the foamed cells aremade even during the period that the resin mixture is retained.

(Extruding Step)

The extruding step S3 is a step of extruding a parison from a die slitby use of a ring-shaped piston. That is, after a predetermined amount ofthe resin has been retained at the cylinder-shaped space 29, the parison(not illustrated) is discharged through the die slit 21 by pressing thering-shaped piston 22 downward.

In the process for producing a blow-molded foam according to thisembodiment, since a system in which the ring-shaped piston 22 extrudesthe parison in the die (an accumulator-inside-die system or anaccumulator head system) is adopted, it is possible to shorten thedistance of the die slit 21, and also to increase the extrusion rate.Thus, it becomes possible to maintain the state of the foamed cells.

On the other hand, a conventional extrusion head illustrated in FIG. 9has a system in which the parison is extruded using an accumulator 35placed outside the die (an accumulator-outside-die system) is adopted,the distance of the die slit is made longer, failing to increase theextrusion rate.

Herein, the extrusion rate of the parison is preferably not less than700 kg/hr. In this case, it is possible to obtain a blow-molded foamhaving excellent surface smoothness. Moreover, the accumulator insidethe die, which is used in the present invention, has an injection rateof not less than 200 cm³/sec, preferably, not less than 500 cm³/sec.

(Molding Step)

The molding step S4 is a step of subjecting the parison to blow moldingby clamping split mold blocks with the parison inserted therebetween andblowing air on the parison.

FIG. 4 is a section view that illustrates a method of blow molding inthe process for producing a blow-molded foam according to the presentinvention.

As illustrated in FIG. 4, the cylinder-shaped parison 32 is extrudedbetween the split mold blocks 33 from the die slit (not illustrated).Then, the split mold blocks 33 are clamped with the parison 32 insertedtherebetween.

Thereafter, air is blown on the parison 32, so that the parison 32 issubjected to blow molding.

Herein, the pressure used for blowing air is preferably 0.05 to 0.15MPa, from the viewpoint of maintaining the shape of foamed cell.

Thus, a blow-molded foam is obtained.

In the process for producing a blow-molded foam according to thisembodiment, by retaining a resin mixture at a predetermined position,the foamed cells can be homogenized in size, and by extruding the resinmixture at a predetermined extrusion rate by using a ring-shaped piston,blow molding is carried out with the size of the foamed cellappropriately maintained.

With this configuration, it becomes possible to obtain a blow-moldedfoam which has homogeneous foamed cells in size, which is light inweight and which is excellent in surface smoothness.

Second Embodiment

The following description is given about a structure in which theblow-molded foam according to the present invention is utilized as askinned panel as a second embodiment of the present invention.

FIG. 5 is a perspective view that illustrates a blow-molded foamaccording to a second embodiment of the present invention.

As illustrated in FIG. 5, a blow-molded foam (hereinafter, referred toalso as “skinned panel”) 3 according to this embodiment has a hollowstructure with double walls, including wall portions that are formed insuch a manner that a thermoplastic resin containing a foaming agentmixed therewith is subjected to blow molding. In this structure, a skinmaterial 4 is bonded to one of surfaces of a base body 2 made of thewall portion. Such a skin material 4 is integrally bonded to the basebody 2, simultaneously with the blow molding of the wall portion in themolding step.

FIG. 6 is a section view of the blow-molded foam illustrated in FIG. 5.

As illustrated in FIG. 6, in the skinned panel 3, the base body 2 madeof the wall portion has a hollow structure with double walls, with ahollow portion 5 included therein, and a plurality of reinforcing ribs 6are formed so as to divide the hollow portion 5. The reinforcing ribs 6make it possible to improve the strength in a vertical direction.

Upon the clamping of the parison in the molding step, the reinforcingribs 6 are formed by pressing the parison wall portion onto the parisonside face in a manner so as to be folded down, by a slide core having aprotruding shape in one direction.

Therefore, in the production of the skinned panel, in the molding step,the reinforcing ribs 6 are also formed simultaneously with the bondingof the skin material 4.

Herein, the wall portion is synonymous with the “wall portion” in theblow-molded foam according to the first embodiment, and since thestructures and physical properties thereof are also the same, thedescription thereof will be omitted.

Moreover, the process for producing the blow-molded foam according tothe second embodiment is the same as the process for producing theblow-molded foam according to the first embodiment, except for theforegoing difference in the molding step.

In the skinned panel 3 (blow-molded foam) according to this embodiment,the wall portion has a closed cell structure in which a plurality offoamed cells are formed. When the expansion ratio of the wall portion isset in a predetermined range, it is possible to provide a lightweightstructure. When the standard deviation of the cell diameter of thefoamed cell in the thickness direction of the wall portion is set in apredetermined range, it is possible to provide the homogeneous foamedcells in size. When the center-line average surface roughness Ra on theouter face of the wall portion is set in a predetermined range, it ispossible to achieve high surface smoothness.

Moreover, it is possible to improve the deposition strength of thereinforcing ribs formed inside the wall face of the panel and thedeposition strength of the skin bonded outside the wall face of thepanel. Further, it is also possible to provide high rigidity andexcellent outer appearance.

EXAMPLES

The following description is given about the present invention morespecifically based upon examples and comparative examples; however, thepresent invention is not intended to be limited by the followingexamples.

Example 1

A propylene homopolymer (thermoplastic resin: trade name: PF814, made bySunAllomer) (70 wt %) having an MFR of 3.0 g/min at 230° C., with along-chain branched structure introduced therein, and a crystallineethylene-propylene block copolymer (NOVATECH PP EC9, made by JapanPolychem) (30 wt %) having an MFR of 0.5 g/min at 230° C. were mixed toform a mixture, and 96 parts by weight of this mixture, 3 parts byweight of talc MB (master batch) serving as a core agent and 1 part byweight of black MB (master batch) serving as a colorant were mixed. Thedensity of the resin mixture was 0.91 g/cm³.

To this was added carbon dioxide gas in a supercritical state as afoaming agent to be foamed so that a resin mixture was prepared. Afterhaving been mixed in an extruder, the resin mixture was retained at acylinder-shaped space defined between a mandrel and a die outer cylinderby using an extrusion head illustrated in FIG. 3, and a cylinder-shapedparison was extruded between split mold blocks illustrated in FIG. 4 ata rate of 1500 kg/hr, by using a ring-shaped piston (accumulator insidedie), and after clamping, air was blown on the parison subjected toclamping, at a pressure of 0.1 MPa, so that a sample A subjected to blowmolding was obtained. Herein, the MFR was measured in conformity withJIS K-7210, with a test load of 2.16 kg being applied thereto.

Example 2

The same processes as those of Example 1 were carried out except thatthe extrusion rate was changed to 750 kg/hr so that a sample B wasobtained.

Example 3

The same processes as those of Example 1 were carried out except that inplace of the carbon dioxide gas, nitrogen gas was used so that a sampleC was obtained.

Example 4

The same processes as those of Example 1 were carried out except that inplace of the carbon dioxide gas, nitrogen gas was used, and that theextrusion rate was changed to 700 kg/hr so that a sample D was obtained.

Example 5

The same processes as those of Example 1 were carried out except that inplace of the carbon dioxide gas, nitrogen gas was used, and that theextrusion rate was changed to 600 kg/hr so that a sample E was obtained.

Example 6

The same processes as those of Example 1 were carried out except thatthe extrusion rate was changed to 600 kg/hr so that a sample F wasobtained.

Comparative Example 1

In place of the extrusion head illustrated in FIG. 3, a conventionalextrusion head illustrated in FIG. 9 was used. That is, the resinmixture that had been mixed in an extruder was supplied to a cross headby using a plunger, from an accumulator cylinder (accumulator outsidedie) in the horizontal direction, installed outside the die head, andextruded as a cylinder-shaped parison through a die slit. Moreover, theextrusion rate was set to 450 kg/hr.

Except for the above-mentioned points, the same processes as those ofExample 1 were carried out so that a sample G was obtained.

Comparative Example 2

The same processes as those of Comparative Example 1 were carried outexcept that in place of the carbon dioxide gas, nitrogen gas was used sothat a sample H was obtained.

Comparative Example 3

The same processes as those of Comparative Example 1 were carried outexcept that the extrusion rate was changed to 300 kg/hr so that a sampleI was obtained.

The physical properties of the samples A to I obtained in Examples 1 to6 and Comparative Examples 1 to 3 were evaluated as described below. Ineach of the samples A to I, comparatively flat portions of three points,that is, two ends and the center portion in the longitudinal direction,were cut off by a microtome (RM2145, made by LEICA), and the cut-offcross section was photographed by a CCD camera (VH-630, made byKeyence).

1. Average Thickness (mm)

In each of samples A to I, thicknesses were measured from thephotographs with respect to the 3 points photographed by the CCD camera,and an average value of the respective values was calculated.

2. Expansion Ratio

The expansion ratio was calculated by dividing the density of each ofresin mixtures used in the samples A to I by the apparent density of thecorresponding wall faces of the samples A to I.

3. Average Cell Diameter (μm)

With respect to the 3 points photographed by the CCD camera in each ofthe samples A to I, the sizes of cell diameters in the thicknessdirection at 5 points taken with equal intervals from the outside to theinside in the thickness direction of the wall face were measured fromthe photograph, and an average value was calculated.

4. Center-Line Average Surface Roughness (Ra) (μm)

In conformity with JIS B0601, the center-line average surface roughnessof each of the samples A to I was measured by using a surface-roughnessmeasuring device (SURFCOM 470A, made by Tokyo Seimitsu Co., Ltd.). Withrespect to the measuring points of the surface roughness of ablow-molded foam, 5 points on the outside of the wall face and 5 pointson the inside of the wall face of the blow-molded foam were measured,and an average value was obtained.

5. Standard Deviation of Cell Diameter (μm)

From the values of the cell diameters in the thickness direction oftotal 15 points measured upon calculating the average cell diameter, thestandard deviation was calculated and found.

Table 1 shows the results obtained by these evaluations.

TABLE 1 Center-line Standard Average average deviation Parison Averagecell surface of cell supply thickness Expansion diameter roughnessdiameter Sample system (mm) ratio (μm) (μm) (μm) Example 1 A Accumulator2.2 2.5 80 4.7 20 inside die Example 2 B Accumulator 2.0 2.1 85 5.2 31inside die Example 3 C Accumulator 1.8 2.4 56 3.8 12 inside die Example4 D Accumulator 2.1 2.7 96 6.1 28 inside die Example 5 E Accumulator 2.52.8 135 8.1 43 inside die Example 6 F Accumulator 2.4 2.7 140 8.8 47inside die Comparative G Accumulator 1.9 2.1 160 10.8 61 Example 1outside die Comparative H Accumulator 2.0 2.5 151 10.2 55 Example 2outside die Comparative I Accumulator 2.3 2.3 177 12.0 71 Example 3outside die

In the samples A to F of Examples 1 to 6, by using the accumulatorinside the die, which had a high injection rate, a parison could beextruded in a short period of time so that it was possible to obtain ablow-molded foam with a small standard deviation of the cell diameter offoamed cells (with less deviations in the distribution of celldiameters) and high surface smoothness.

Moreover, it was found that, by using nitrogen in a supercritical stateas a foaming agent, the diameter of the foamed cells could be madesmaller.

In contrast, in the samples G to I of Comparative Examples 1 to 3,deviations occurred in the distribution of the cell diameters of foamedcells. This is presumably caused by the fact that, in the case where theaccumulator outside the die is used, since, upon extrusion of athermoplastic resin in a fused state stored in the cylinder, thethermoplastic resin is extruded, with its flowing direction beingchanged by 90 degrees at the cross-head portion, and since the distancefrom the cylinder placed outside the die to the die slit from which theparison is extruded becomes comparatively longer, the pressure loss ofthe thermoplastic resin to be extruded becomes greater.

Next, the effects of the samples A to I obtained by Examples 1 to 6 andComparative Examples 1 to 3 were evaluated as described below:

1. Photographs

In the samples A and G obtained in Example 1 and Comparative Example 1,comparatively flat portions in the center portion in the longitudinaldirection were cut off by a microtome (RM2145, made by LEICA), and thecut-off cross section was photographed by a CCD camera (VH-630, made byKeyence).

FIG. 7 illustrates a photograph of the sample A of Example 1, thusobtained, and FIG. 8 illustrates a photograph of the sample G ofComparative Example 1, thus obtained.

2. Peel Strength (gf)

To a test piece cut off from each of the samples A to I was adhered anon-woven fabric (hereinafter, referred to as “packing”) (thickness: 3mm, width: 10 mm) to which double-sided tapes had been bonded, and oneof the end faces of the packing was adhered and secured to the testpiece, with the other end face being attached to a tensile testingmachine.

Moreover, the other end face of the packing was pulled by the tensiletesting machine in a manner so as to be folded over to the other endface side, and the peel strength at this time was measured. Herein, asthe packing, a non-woven fabric/acryl-based adhesive material (interiormember securing double-sided tape #5782, made by Sekisui Chemical Co.,Ltd.) having an adhesive strength of 18.6 N/25 mm measured in conformitywith JIS Z0237 (180° peeling method) was used, and the tension rate wasset to 300 mm/min.

Table 2 shows the results of the tests.

3. Outer Appearance

The outer appearance of each of the samples A to I was visuallyevaluated in accordance with the following criteria.

-   Excellent: The surface was smooth, with a uniform outer appearance.-   Moderate: Although the surface was comparatively smooth, the outer    appearance was inferior in uniformity.-   Bad: Surface irregularities were clearly observed visually, and the    outer appearance was inferior.

Table 2 shows the results of the tests.

TABLE 2 Peel Strength (gf) Appearance Example 1 Sample A 260 ExcellentExample 2 Sample B 250 Excellent Example 3 Sample C 270 ExcellentExample 4 Sample D 230 Moderate Example 5 Sample E 210 Moderate Example6 Sample F 200 Moderate Comparative Sample G 170 Bad Example 1Comparative Sample H 180 Bad Example 2 Comparative Sample I 155 BadExample 3

In the samples A to F of Examples 1 to 6 of the present invention, thepeel strength was excellent in comparison with those of the samples G toI of Comparative Examples 1 to 3. Thus, the blow-molded foam of thepresent invention was confirmed to be excellent in smoothness.

In view of these facts, it was confirmed that the present inventioncould provide a blow-molded foam which has homogeneous foamed cells insize, is light in weight and is excellent in surface smoothness.

INDUSTRIAL APPLICABILITY

In addition to a climate control duct for vehicles and a skinned panel,the blow-molded foam of the present invention is applicable to aninterior material for vehicles, and the like. The blow-molded foam makesit possible to devote to weight reduction of a vehicle without causingdegradation of various physical properties as plastic parts.

DESCRIPTION OF REFERENCE SIGNS

-   1 . . . Climate control duct (blow-molded foam)-   2 . . . Base body-   3 . . . Skinned panel (blow-molded foam)-   4 . . . Skin material-   5 . . . Hollow portion-   6 . . . Reinforcing rib-   11 . . . Main body-   12 . . . Air outlet-   13 . . . Air inlet-   20 . . . Extrusion head-   21 . . . Die slit-   22 . . . Ring-shaped piston-   27 . . . Mandrel-   28 . . . Die outer cylinder-   29 . . . Cylinder-shaped space-   32 . . . Parison-   33 . . . Split mold blocks-   35 . . . Accumulator-   51 . . . Mixing step-   S2 . . . Retaining step-   S3 . . . Extruding step-   S4 . . . Molding step

1. A process for producing a blow-molded foam including foamed cellshaving a flat shape in a direction orthogonal to a thickness directionof a wall defining the blow-molded foam, where an average cell diameterof the foamed cell in the thickness direction of the wall is 56 μm ormore and less than 300 μm, and a standard deviation of a cell diameterof the foamed cell in the thickness direction of the wall is less than40 μm, the process comprising: kneading a polyolefin-based resincontaining a foaming agent mixed therewith in an extruder; retaining thepolyolefin-based resin at a cylinder-shaped space defined between amandrel and a die outer cylinder; and extruding a parison between splitmold blocks from a die slit at a extrusion rate of 700 kg/h or more byuse of a ring-shaped piston after a predetermined amount of the resin isretained at the cylinder-shaped space and carrying out blow moldingafter clamping by blowing air on the parison at a pressure of 0.05 to0.15 MPa.
 2. The process for producing a blow-molded foam according toclaim 1, wherein a supercritical fluid is used as the foaming agent. 3.The process for producing a blow-molded foam according to claim 2,wherein a nitrogen gas is used as the foaming agent.